Optical concentration obtained by light confinement bears unique features that can increase the efficiency of a photochemical reactor. A suitable implementation of this method for a solar reactor is a series of parallel tubular receivers sealed in a slab-shape reflective cavity, in which light is trapped thanks to a self-adaptive optical filtering mechanism. To predict the concentration in such a generic configuration, we had previously established an analytical model based on idealistic assumptions, which are not valid in our real configuration. Here, we use analytical calculations and numerical ray-trace simulations to investigate how the finite size of the latter impacts the prediction of our model and extrapolate design guidelines for minimal departure from ideality. We apply these guidelines to design an optical concentrator maximizing flux density on tubular receivers and discuss the upper bound to the method, as well as the benefits from its unique features. Accounting for practical and technological limitations, this method can provide optical concentration in the order of ten suns in our generic configuration.

Finding the proper trade-off between blocking direct sunlight, ensuring sufficient indoor daylighting and view out is a particularly delicate task especially in arid climates, due to harsh environmental conditions. As a tentative answer to this challenge, an adaptive shading and daylighting system (Shape Variable Mashrabiya – SVM) has been developed by the authors, described in an earlier paper. In this paper, we analyze how the SVM may affect annual lighting and global primary energy performance of an office building in Abu Dhabi: the SVM was applied to east and west façades and compared to external Venetian blinds, reflective and selective glazing.

The design of a solar protection system that can minimize solar gains while maximizing daylight and view to the outside is particularly challenging in arid climates, such as in the Middle-East, where sand, wind and corrosion impose specific constraints. We propose a system that provides a trade-off for three requirements: (i) maximize diffuse sunlight and view to the outside, (ii) efficiently block direct sunlight and (iii) transform a fraction of it into diffuse light for indoor daylighting. Compliance with this last requirement provides a solution for the common problem of insufficient daylighting even in the presence of abundant solar radiation, which often forces occupants to fully close their shading system and use electric lighting. In addition, our design potentially copes well with these extreme environmental conditions and preserves local architectural character (mashrabiya-inspired design). In this paper, we establish quantitative specifications for these three requirements, provide the working principle of our shading and daylighting system and its design, which consists of a shape variable mashrabiya (SVM). We calculate and analyze the annual daylighting performance of our SVM and benchmark it against the performance of Venetian blinds and diffuse sunlight alone. Finally, we provide the minimum reflectance required for the SVM to comply with our third requirement. We built a mock-up of our SVM to investigate the validity of our simulation model.

Our novel stationary solar concentrator, which relies on a new self-adaptive optical valve, theoretically allows a concentration up to 9.5 suns, which is 4.5× higher than the limit of the étendue law, and provides perfect illumination homogeneity.

We briefly categorize and compare parallel goniophotometers, which are instruments capable of simultaneously measuring the far-field distribution of light scattered by a surface or emitted by a source over a large solid angle. Little is known about the accuracy and reliability of an appealing category, the catadioptric parallel goniophotometers (CPGs), which exploit a curved reflector and a lens system. We analyzed the working principle common to all the different design configurations of a CPG and established the specifications implicitly imposed on the lens system. Based on heuristic considerations, we show that the properties of a real (thick) lens system are not fully compatible with these specifications. This causes a bias to the measurements that increases with the acceptance angle of the lens system. Depending on the angular field, the measured sample area can be drastically reduced and shifted relative to the center of the sample. To gain insights into the nature and importance of the measurement bias, it was calculated with our model implemented in MATLAB for the CPG configuration incorporating a lens system with a very large acceptance angle (fisheye lens). Our results demonstrate that, due to the spatio-angular-filtering properties of the fisheye lens, this category of CPGs is so severely biased as to give unusable measurements. In addition, our findings raise the question of the importance of the bias in the other types of CPGs that rely on a lens system with a lower acceptance angle.

A thorough knowledge of the angular distribution of light scattered by an illuminated surface under different angles is essential in numerous industrial and research applications. Traditionally, the angular distribution of a reflected or transmitted light flux as function of the illumination angle, described by the Bidirectional Scattering Distribution Function (BSDF), is measured with a point-by-point scanning goniophotometer yielding impractically long acquisition times. Significantly faster measurements can be achieved by a device capable of simultaneously imaging the far-field distribution of light scattered by a sample onto a two-dimensional sensor array. Such an angular-to-spatial mapping function can be realized with a parallel catadioptric mapping goniophotometer (CMG). In this contribution, we formally establish the design requirement for a reliable CMG. Based on heuristic considerations we show that, to avoid degrading the angular-to-spatial function, the acceptance angle of the lens system inherent to a CMG must be smaller than 60°. By means of a parametric study, we investigate the practical design limitations of a CMG caused by the constraints imposed by the properties of a real lens system. Our study reveals that the values of the key design parameters of a CMG fall within a relatively small range. This imposes the shape of the ellipsoidal reflector and drastically restricts the room for a design trade-off between the sample size and the angular resolution. We provide a quantitative analysis for the key parameters of a CMG for two relevant cases.

A reliable computer simulation of natural and artificial lighting of an indoor environment requires the thorough knowledge of the angular intensity distribution of light scattered or emitted by the various objects involved such as the illuminated surfaces, the trans-illuminated windows or fenestration systems, as well as the luminaires. The angular intensity distribution of light flux reflected, transmitted or emitted as a function of the illumination angle can be measured with an instrument called goniophotometer. Fast measurement, essential in most practical applications, requires the simultaneous detection of all scattering directions with a so-called parallel goniophotometer. In this paper we define and explain the three working principles on which a parallel goniophotometer can rest, namely (i) screen imaging, (ii) dioptric angular mapping, and (iii) catadioptric angular mapping. We provide a state-of-the-art of these instruments and compare their performance and limitations based on a few key parameters.

Two pairs of identical experimentation modules are being developed within the framework of a collaborative initiative named NOMAD – Network of Optimization Modalities in Architectural Design - between the Interdisciplinary Laboratory of Performance-Integrated Design (LIPID) and the Laboratory of Architecture and Sustainable Technologies (LAST), both at EPFL. NOMAD relates to a series of projects related to the optimization of the building envelope in a sustainability context. Based on comparative evaluations in two climates (cool-temperate versus hot-arid), these projects will most notably rely on a dual infrastructure: the NOMAD modules. These modules will be located in Lausanne, Switzerland and in the Middle East (Ras-Al-Kaimah, United Arab Emirates (U.A.E.). They are currently in design development and will be operational in the fall of 2012 for the launching of the Master in Energy Management and Sustainability Caravans. One pair of modules will be located on the main EPFL campus in Lausanne, Switzerland and the other pair will be installed on the site of the EPFL Middle East campus in Ras-Al-Kaimah, U.A.E. The modules’ dimensions will be 3 m wide x 3 m high x 9 m deep so as to allow deep plan layouts, and include an active insulation skin for temperature and heat exchange control. Having the modules in pairs offers the option either rely on a reliable reference case in any measurement campaign, to hide the measurement setup from the users in field studies, or to conduct two experiments simultaneously to get most out of periods where optimal climate conditions are scarce. This dual climate approach allows us to open up an innovative and original research framework, able to generate new facade concepts with high environmental standards, and to develop bio-climatic strategies based on multiple criteria. This unique dual research facility will thus be particularly adapted to investigate climate-related issues in building technology and to enhance the educational potential in sustainable architecture at EPFL and EPFL Middle East.

We present a semi-analytical model of optical coherence tomography (OCT) taking into account multiple scattering. The model rests on the assumptions that the measured portion of the backscattercd sample field is spatially coherent and that the sample is motionless relative to measurement time. This allows modeling an OCT signal as a sum of spatially coherent fields with random phase arguments - constant during measurement time - caused by multiple scattering. We calculate the mean OCT signal from classical results of statistical optics and a Monte Carlo simulation. Our model is shown to be in very good agreement with a whole range of experimental data gathered in a comprehensive study of cross-talk in wide-field OCT realized with spatially coherent illumination. The study consists of depth scan measurements of a mirror covered with an aqueous suspension of microspheres. We investigate the dependence of cross-talk on important optical system parameters, as well as on some relevant sample properties. We discuss the more complex OCT models based on the extended Huygens-Fresnel principle, which rest on different assumptions since they assume partially coherent interfering fields.

The two previously reported calculations of the amplitude distribution of speckles in optical coherence tomography, each based on a different mathematical formulation, yield different results. We show that a modification of an initial assumption in one of the formulations leads to equivalent results.

We present a comprehensive study of multiple-scattering effects in wide-field optical coherence tomography (OCT) realized with spatially coherent illumination. Imaging a sample made of a cleaved mirror embedded in an aqueous suspension of microspheres revealed that, despite temporal coherence gating, multiple scattering can induce significant coherent optical cross talk. The latter is a serious limitation to the method, since it prevents shot-noise-limited detection and diffraction-limited imaging in scattering samples. We investigate the dependence of cross talk on important system design parameters, as well as on some relevant sample properties. The agreement between theoretical and experimental results for the wide range of parameters investigated was very good, in both the lateral and the axial dimensions. This further confirms the validity of the model developed in our companion paper [J. Opt. Soc. Am. A 22, 1369-1379 (2005)].

We present a new model of optical coherence tomography (OCT) taking into account multiple scattering. A theoretical analysis and experimental investigation reveals that in OCT, despite multiple scattering, the field backscattered from the sample is generally spatially coherent and that the resulting interference signal with the reference field is stationary relative to measurement time. On the basis of this result, we model an OCT signal as a sum of spatially coherent fields with random-phase arguments--constant during measurement time--caused by multiple scattering. We calculate the mean of such a random signal from classical results of statistical optics and a Monte Carlo simulation. OCT signals predicted by our model are in very good agreement with a depth scan measurement of a sample consisting of a mirror covered with an aqueous suspension of microspheres. We discuss other comprehensive OCT models based on the extended Huygens-Fresnel principle, which rest on the assumption of partially coherent interfering fields.

Current spectroscopic optical coherence tomography (OCT) methods rely on a posteriori numerical calculation. We present an experimental alternative for accessing spectroscopic information in OCT without post-processing based on wavelength de-multiplexing and parallel detection using a diffraction grating and a smart pixel detector array. Both a conventional A-scan with high axial resolution and the spectrally resolved measurement are acquired simultaneously. A proof-of-principle demonstration is given on a dynamically changing absorbing sample. The method's potential for fast spectroscopic OCT imaging is discussed. The spectral measurements obtained with this approach are insensitive to scan non-linearities or sample movements.

Comparison of two illumination modes for wide-field optical coherence tomography has revealed that spatially coherent illumination generates coherent cross talk, causing significant image degradation, and that spatially incoherent illumination, with an adequate interferometer design, provides an efficient mechanism for suppression of coherent cross talk. This is shown by comparison of a pulsed laser with a thermal light source for a U.S. Air Force resolution target covered with a scattering solution made from microbeads as well as for an ex vivo tooth.

A simple method for the calibration of optical path difference modulation in low-coherence interferometry is presented. Spectrally filtering a part of the detected interference signal results in a high-coherence signal that encodes the scan imperfections and permits their correction. The method is self-referenced in the sense that no secondary high-coherence light source is necessary. Using a spectrometer setup for spectral filtering allows for flexibility in both the choice of calibration wavelength and the maximum scan range. To demonstrate the method's usefulness, it is combined with a recently published digital spectral shaping technique to measure the thickness of a pellicle beam splitter with a white-light source.

We demonstrate phase space tomography for the measurement of the transversal spatial coherence function of light after propagation through a scattering medium. The results of this approach are compared to measurements performed with shearing-interferometry. Implications for parallel Optical Coherence Tomography will be briefly discussed.

Most current optical coherence tomography systems provide two-dimensional cross-sectional or en face images. Successive adjacent images have to be acquired to reconstruct three-dimensional objects, which can be time consuming. Here we demonstrate three-dimensional optical coherence tomography (3D OCT) at video rate. A 58 by 58 smart-pixel detector array was employed. A sample volume of 210x210x80 m3 (corresponding to 58x58x58 voxels) was imaged at 25 Hz. The longitudinal and transverse resolutions are 3 m and 9 m respectively. The sensitivity of the system was 76 dB. Video rate 3D OCT is illustrated by movies of a strand of hair undergoing fast thermal damage.